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<h2><a name="SECTION0023210000000000000000">
21.2.1 Background, details, hints, tips and caveats</a>
</h2>

<p>
The C standard defines the locale as a program-wide property that may
be relatively expensive to change.  On top of that, some
implementation are broken in such a way that frequent locale changes
may cause core dumps.  This makes the locale somewhat painful to use
correctly.

<p>
Initially, when a program is started, the locale is the "<tt class="samp">C</tt>" locale, no
matter what the user's preferred locale is.  The program must
explicitly say that it wants the user's preferred locale settings by
calling <code>setlocale(LC_ALL, '')</code>.

<p>
It is generally a bad idea to call <tt class="function">setlocale()</tt> in some library
routine, since as a side effect it affects the entire program.  Saving
and restoring it is almost as bad: it is expensive and affects other
threads that happen to run before the settings have been restored.

<p>
If, when coding a module for general use, you need a locale
independent version of an operation that is affected by the locale
(such as <tt class="function">string.lower()</tt>, or certain formats used with
<tt class="function">time.strftime()</tt>), you will have to find a way to do it
without using the standard library routine.  Even better is convincing
yourself that using locale settings is okay.  Only as a last resort
should you document that your module is not compatible with
non-"<tt class="samp">C</tt>" locale settings.

<p>
The case conversion functions in the
<tt class="module"><a href="module-string.html">string</a></tt><a id='l2h-4889' xml:id='l2h-4889'></a> module are affected by the
locale settings.  When a call to the <tt class="function">setlocale()</tt> function
changes the <tt class="constant">LC_CTYPE</tt> settings, the variables
<code>string.lowercase</code>, <code>string.uppercase</code> and
<code>string.letters</code> are recalculated.  Note that code that uses
these variable through `<tt class="keyword">from</tt> ... <tt class="keyword">import</tt> ...',
e.g. <code>from string import letters</code>, is not affected by subsequent
<tt class="function">setlocale()</tt> calls.

<p>
The only way to perform numeric operations according to the locale
is to use the special functions defined by this module:
<tt class="function">atof()</tt>, <tt class="function">atoi()</tt>, <tt class="function">format()</tt>,
<tt class="function">str()</tt>.

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<span class="release-info">Release 2.5.1, documentation updated on 18th April, 2007.</span>
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